1. Field of the Invention
The present invention relates to a heating device for fixing electrostatically deposited toner on a paper sheet in a photocopying machine, or for heating a plastic sheet for a film laminating machine.
2. Description of the Related Art
Heating devices used for the above purposes are disclosed in Japanese Patent Application Laid-open No. 2-59356 or in Japanese Patent Application Laid-open No. 2-65086 for example. Such a heating device includes a strip-like heating resistor formed on a substrate made of a heat-resistant insulating material such as ceramic for example, and a protective glass coating formed on the substrate to cover the heating resistor layer. Typically, the protective glass coating is designed to withstand the heat generated at the heating resistor for electrical insulation while also preventing the heating resistor from being worn out due to direct contact with a sheet material.
In such a heating device, it is necessary to insure a sufficient electrical insulation, since a considerably large current is passed through the heating resistor layer to generate Joule heat for heating the sheet material. However, a conventional glass material used for the protective glass coating generally has a dielectric strength of only about 14-15 volts per a thickness of 1 .mu.m. Thus, it is necessary to make the thickness of the protective glass coating considerably large for insuring a sufficient electric insulation. As a result, in the conventional heating device, the heat capacity of the protective glass coating becomes large, so that the thermal response at the surface of the protective glass coating is likely to deteriorate (the temperature rises slowly). If, to compensate for this, the amount of the heat generated at the heating resistor is increased, a problem of wasting energy will occur due to low thermal efficiency.
In view of the above problem, PCT Publication No. WO96/31089 (corresponding to U.S. patent application Ser. No. 08/732,351 filed Mar. 25, 1996) discloses a heating device which incorporates a protective glass coating containing an alumina powder filler in a proportion of 3.about.30 wt %. The alumina powder filler has an average grain size of up to 5 .mu.m. The addition of the alumina powder as a filler doubles the dielectric strength of the protective glass coating per unit thickness when compared with a protective glass coating which does not contain any alumina powder. Thus, the protective glass coating may be considerably reduced in thickness for improving the thermal response (namely, heat transmission) of the glass coating.
However, it has been experimentally found that the dielectric strength of the protective glass coating no longer increases even if the alumina powder is added in excess of 30 wt %. In fact, the dielectric strength of the protective glass coating starts decreasing when the alumina powder is added beyond 30 wt %.
The inventor of the present invention has carried out research as to causes for the lowering of dielectric strength when the alumina powder is added in excess of 30 wt %. As a result, the inventor has found that the dielectric strength decrease is attributable to foams trapped in the glass coating, as illustrated in FIG. 6 of the accompanying drawings. In FIG. 6, reference character A designates alumina grains, whereas the foams are denoted by reference character B.
More specifically, if the content of the alumina powder is increased beyond 30 wt %, the apparent fluidity of the glass material lowers because the softening point of alumina is higher than that of the glass material, so that the lowered fluidity of the glass material hinders escape of gas. Further, when the grain size of the added alumina powder is as large as 5 .mu.m, inside gas tends to stay in the shade of the alumina grains.
Moreover, when alumina power having a relatively large grain size is added in excess of 30 wt %, part of the alumina grains are exposed at the surface of the protective glass coating, as also shown in FIG. 6. As a result, the surface of the glass coating is roughened and fails to provide smooth contact with a sheet material.